This invention relates to an image data processing method, and in particular, to an image data processing method suitable to an ink jet printer.
In recent years, an ink jet printer has been rapidly developing for its simple structure and capability of forming a high-quality image. Incidentally, in an ink jet printer, for the purpose of making its image quality higher, sometimes inks of plural densities having the same hue are used. FIG. 5 is a drawing showing a relationship between gradation and spatial ink density in the case where inks of two colors, a deep color and a pale color, are used.
As described in the above, by making a pale-colored ink included, it becomes possible to improve the image quality at lower-density side; however, according to the conventional technology shown in FIG. 5(a), spatial density of pale ink drops (proportion of the area occupied by ink drops to the ink landing domain) has already reached to 100% at the half point in the gray-level scale. In a general ink jet printer, a desired color is formed by making inks of 4 colors land on a medium; however, in such a situation that spatial density of ink drops reaches to 100% even when the half of the gray levels have been expressed by using an ink of one color, even though the absorption characteristic is improved, there is some concern that ink overflow is easy to occur.
For such a problem, as shown in FIG. 5(b), it is considered to reduce the spatial density of an ink by mixing it with a deep-colored ink before the spatial density of the pale-colored ink dots reaches to 100% (refer to the publications of the unexamined Japanese patent application H1-128836 and H9-156127). However, in order to achieve it, for example, it is necessary to apply an error diffusion processing to a deep-colored ink and a pale-colored ink independently, which poses a problem that the processing circuit becomes larger-scaled, or it becomes impossible to obtain the desired high image quality.
This invention has been made in view of the above-mentioned problems of conventional technologies, and it is its object to provide an image data processing method for an ink jet printer having a simple structure and being capable of suppressing ink overflow.
An image data processing method for ink jet printer of this invention comprises the steps of obtaining an allowable amount of ink in accordance with inputted image data and the characteristic of a medium for forming an image, selecting an ink jetting domain on the basis of said allowable amount of ink, while fixing the amount of ink in said selected ink jetting domain at a specified value, and practicing a quantization processing with the amount of ink in said ink jetting domain fixed at the specified value. Owing to this, in the case where there is some concern of ink overflow in the medium in accordance with the inputted image data and the characteristics of a medium for forming an image, in order to limit the amount of ink to be jetted to a value under an allowable amount of ink which is obtained from the above-mentioned image data and characteristic of the medium, an ink jetting domain where the amount of ink is determined to be a specified value (a low value or zero) is forcibly formed. On the other hand, the color error produced by forcibly forming a domain where the amount of ink is determined to be a specified value is canceled by adjusting the color in the neighboring domains by a quantization processing such as an error diffusion processing, by which a high-quality image can be formed with a simple structure, while ink overflow is suppressed.
The principle of this invention will be explained in detail by using a drawing. FIG. 1 is a drawing showing ink jetting domains on a medium schematically. In FIG. 1, positions indicated by a circle are ink landing (jetting) domains. In the drawing, the zone A1 enclosed by the dotted line is composed of 9 ink landing domains, and in the case where ink overflow in zone A1 is surmised from the inputted image data and the characteristics of the medium for forming an image, the amount of ink jetting for the central ink landing domain A2 is made to be a specified value, for example, zero. By doing this, even if landing of ink drops exceeding the absorption limit of the medium occurs in the neighborhood of the domain A2, the domain A2 absorbs the overflow; therefore, as the result, ink overflow can be suppressed. However, if the amount of ink of the domain A2 is made zero, the color of the domain A2 is recognized as the color of the medium (generally speaking, white), and if any control is not made, the image color of the zone A1 is reduced to a whitish color. Therefore, such a color error is canceled by making ink drops having a deeper color land on the ink landing domains around the zone A2. The degree of making the ink color deep can be obtained, for example, by an error diffusion processing, but it is not limited to this. Because the ink landing domains are extremely minute, when it is viewed from a distant position, the color of the zone A1 appears approximately the same as that of the original image; hence, the image quality can be preserved.
As described in the above, it is desirable to make the above-mentioned specified value zero, because it can suppress ink overflow effectively.
Further, it is desirable to make the above-mentioned quantization processing an error diffusion processing, because the correction of color error can be easily made.
Further, it is desirable to determine the aforesaid allowable amount of ink in accordance with the kind of the ink to be used for expressing the color corresponding to the inputted image data. The reason is that, for some color of ink, the degree of contribution to the color density are different from other colors.
Further, it is desirable that the aforesaid processing to fix the amount of ink at a specified value is determined in accordance with the kinds of the ink color to be used for expressing the color corresponding to the inputted image data. This can be coped with by changing the characteristic of the noise and/or the characteristic of the diffusion filter in the quantization processing in calculating the specified value.
Further, it is desirable to make the aforesaid kinds of ink color include at least cyan, magenta, and yellow because this can be applied to a usual ink jet printer.
Further, as regards the aforesaid at least one kind of ink color, comprises a pale color and a deep color of the same hue, this invention can be more effectively applied, because ink overflow tends to occur.
Further, it is desirable that the aforesaid characteristics of a medium include the ink absorption characteristic of said medium. The reason is that some medium is made of a material which is easy to produce ink overflow.
Further, it is desirable to change the characteristic of the noise in the quantization processing in calculating the specified value in accordance with the color corresponding to the inputted image data.
Furthermore, it is desirable to change the characteristic of the diffusion filter in the quantization processing in calculating the specified value in accordance with the color corresponding to the inputted image data.